Control of hand orientation and arm movement during reach and grasp.

We studied the coordination of arm and wrist motion in a task requiring fine control of hand orientation. Subjects were instructed to reach and grasp one of two targets positioned in the frontal plane at various orientations. The task was performed under three target conditions: fixed orientation, predictably perturbed orientation, and randomly perturbed orientation. For fixed target orientations, the hand began to rotate to the required orientation from the beginning of the reach. Hand peak supination angles scaled linearly with target orientations. The trajectories of hand/arm joint angles also had a one-to-one relationship with different target orientations. These demonstrate that target orientation is a constraint on the hand/arm final orientation, a control variable to be specified in advance by the central nervous system (CNS). Under perturbation conditions, subjects were still able to complete the task smoothly. In the early trials of the predictable perturbation, the hand rotated first to the original target orientation and then corrected for the final target orientation. Initial corrections occurred about 200 ms after the onset of perturbation. This latency decreased as the subjects adapted to the perturbation, and the hand orientation trajectory shifted to match the unperturbed trajectory for the final orientation. By contrast, we observed no clear changes in orientation trajectory under the randomly perturbed conditions. These suggest that feedback control is important to the execution of the movement, but that the CNS tends to optimize feedforward planning rather than feedback correction when the disturbance information is predictable.